1. Field of the Invention
The present invention relates generally to gear transmissions and more particularly to mechanisms for compensating axial thrust forces between gears in a high speed environment.
2. Brief Description of the Prior Art
In gear trains which employed helical gears, axial thrust was generated on the respective gear shafts due to the driving engagement between the gears. The direction of axial thrust generated in the respective gear shafts was dependent upon the directions of rotation of the respective shafts as well as the driving or driven function which the respective shafts assumed. In relatively low speed gearings, axial thrust bearings have been employed to compensate for the axial thrusts which have been generated.
In high speed gearings, however, thrust collars have been employed to absorb and compensate axial thrust forces. The employment of thrust collars was required at high tooth speeds (approximately 130 meters per minute or greater) in lieu of axial thrust bearings because thrust bearings generated excessive heat at such speeds.
A typical installation suitable for the employment of thrust collars has been gearing systems for high speed compressor installations where, for example, tooth speeds in the order of 178 meters per minute at 13,400 revolutions per minute were encountered. In such systems, the transmitted power was in the order of 17 megawatts (approximately 20,000 horsepower).
In construction, the helical gears were normally formed in one piece with their respective shafts and a pair of annular thrust collars were force fitted on one shaft against opposite sides of the gear. Journal bearings were located on opposite sides of the shaft with the thrust collars between the bearings and the gear. The thrust collars were of a diameter greater than that of the gear on its shaft and served to frame the meshing gear to thereby prevent axial movement between the respective gears.
This prior thrust collar design was subject to certain disadvantages. At the high rotational speeds, centrifugal force counteracted the force fit engagement between the thrust collars and their shaft. Thus, the thrust collars were unable to absorb the axial thrusts generated in the system. A typical speed at which the force fit engagement between the thrust collars and their shaft has broken down due to centrifugal forces generated has been at a pitch speed in the order of approximately 150 meters per second.
One approach toward overcoming this problem has been to reduce the circumferential speeds by reducing the diameters of the respective gears and their shafts. This, however, resulted in the requirement of increasing the width of the respective gears in order to maintain the requisite strength required for power transmission. Unfortunately, a result of this approach has been that, when the gears were widened, excessive flexure and twist were encountered as a result of the increased space between the shaft supports. The increased flexure and torque were accentuated due to the fact that the thrust collars were positioned between the gear and the journal bearings.
Attempts at resolving this problem by compensating for the excessive torque with a modified tooth angle only compounded the situation. This was because, when the gear train operated under a partial load, the modified tooth angle resulted in a unilateral tooth load.
Among the further disadvantages of the prior thrust collar assemblies was that they were often difficult to install on driving shafts due to the flanges which were normally forged on the driven shaft.
A further problem which has been encountered with the use of prior thrust collar assemblies has been that the thrust collars have hindered oil and heat transfer between the teeth of the respective gears. This was partially due to the fact that the collars provided a peripheral lip about one of the gears and framed the teeth of the other gear.